Internal combustion engines include cylinder heads in which hydraulic lash adjusters are typically arranged which have hydraulic fluid galleries that feed pressurized hydraulic fluid, typically at a pressure of at least 0.5 bar, to a port defined in a housing of the hydraulic lash adjuster. A plunger is arranged within a bore of the housing and supports an end of a finger follower on a support head thereof. The plunger is axially displaceable to perform the hydraulic lash adjustment. For engines with cylinder deactivation technology, optionally a switching function can be performed by the hydraulic lash adjuster to provide a zero lift opening of the associated valve. When the switching function is provided, the hydraulic lash adjuster is provided as two parts in the form of an inner and outer housing, which are locked together for normal operation, and when deactivation is desired, a switching port of the housing feeds hydraulic fluid from a switching oil gallery, typically at a pressure of at least 1 bar, into a chamber of a locking pin to unlock the inner housing from the outer housing so that the inner housing can reciprocate within the outer housing. This is shown for example in U.S. Pat. No. 8,235,017.
It is also known to use switchable finger followers that have a first, hi-lift and a second, no or low-lift switching mode. Here, the switchable finger followers are activated or deactivated by pressurized hydraulic fluid that is fed via a feed path from the switching oil gallery, through the hydraulic lash adjuster, and to an actuator chamber in the switchable finger follower to actuate a switching function of the finger lever assembly between the first and second valve lift modes. See for example, U.S. Pat. No. 7,909,007.
One known issue with switching oil galleries is that there can be a lag time for actuation of the switching function, whether in the hydraulic lash adjuster or in a switchable finger follower, due to air bubbles in the hydraulic fluid in the switching oil gallery or the switching oil hydraulic fluid path to the actuator, which at a minimum delays the switching time, affecting engine performance.